1. Field of the Invention
This invention relates to exhaust gas heat recovery systems in internal combustion engines, and particularly to an exhaust gas heat recovery system wherein exhaust gases in the internal combustion engine such as a diesel engine, gas turbine and the like are utilized to generate steam which is used as the steam for driving a turbine for an electric generator and the steam for miscellaneous uses.
2. Description of the Prior Art
Heretofore, there has been known an exhaust gas heat recovery system in a diesel engine for use in a ship and the like, wherein steam is generated by an exhaust gas economizer as shown in FIG. 1. The system shown in FIG. 1 principally comprises: an exhaust gas economizer 1 incorporating therein tube nests such as a preheating section 11, an evaporating section 12 and a superheating section 13; a feed water heater 4 for heating feed water fed to a steam separation drum 2 (for cooling circulating feed water fed to the preheating section 11 and the evaporating section 12 of this exhaust gas economizer); a steam separation drum 2 for separating steam from a fluid of steam-water mixture discharged from the evaporating section; and a turbine 21 (which is connected to a generator), to which is fed superheated steam from the superheating section 13.
In the abovedescribed arrangement, the exhaust gas discharged from the internal combustion engine is introduced to the economizer 1 as indicated by an arrow Qg, where it generates steam, the heat quantity of which is recovered. A supply of feed water is fed by a main feed water pump 7 through a surface type feed water heater 4 to the steam separation drum 2, which is common in use as a steam drum for an auxiliary boiler, and thereafter is introduced by a feed water circulating pump 3 to the preheating section 11 of the exhaust gas economizer 1 through the feed water heater 4 and a piping 4A, subsequently, generates steam in the evaporating section 12, and thereafter, is discharged into the main stream separation drum 2 as the fluid of steam-water mixture. A saturated steam separated in the steam separation drum 2 is used as the steam for miscellaneous uses for heating a part of fuel oil and the like, a drain thereof is introduced to a drain tank 26, the remaining part of steam is introduced to the superheating section 13 of the exhaust gas economizer 1 and drives a steam turbine 21 for a turbo-generator as the superheated steam, exhausted steam is condensed into water in the condenser 23, the water is discharged into the drain tank 26 by a condenser pump 24, subsequently, sucked by the main water feed pump 7, and returned to the feed water heater 4.
Changes in temperature at the side of the exhaust gases and at the side of the circulating feed water in the exhaust gas economizer 1 of the conventional system described above are indicated by solid lines in FIG. 2. In FIG. 2, T.sub.i is a temperature at an exhaust gas inlet of the exhaust gas economizer 1, T.sub.0 a temperature at an exhaust gas outlet, T.sub.1 a temperature of the exhaust gases between the preheating section 11 and the evaporating section 12 of the exhaust gas economizer 1, T.sub.2 a temperature of the exhaust gases between the evaporating section 12 and the superheating section 13 of the economizer 1, t.sub.i a temperature at a circulating water inlet of the preheating 11, t.sub.s a saturation temperature of the circulating feed water at the evaporating section 12, t.sub.0 a temperature at a superheated steam outlet of the superheating section 13, .DELTA.t.sub.0 a difference in terminal temperature between the temperature t.sub.i at the circulating feed water inlet of the preheating section 11 and the temperature T.sub.0 at the exhaust gas outlet, and .DELTA.t.sub.1 a difference in terminal temperature between the temperature t.sub.s at the circulating feed water outlet of the preheating section 11 and the temperature T.sub.1 at the exhaust gas outlet of the evaporating section 12.
Now, in order to improve the rate of recovery of the exhaust gas heat in an internal combustion engine, it is necessary to lower the temperature T.sub.0 at the exhaust gas outlet of the exhaust gas economizer 1 as low as possible. When the temperature T.sub.0 at the exhaust gas outlet of the exhaust gas economizer 1 is lowered, the following items must be considered.
(1) It is important to hold a temperature t.sub.i of the circulating feed water at the inlet of the preheatiang section 11 of the exhaust gas economizer 1 to predetermined temperatures or thereabove, and to raise the surface temperature of a tube of the exhaust gas economizer for preventing corrosion by generated sulfuric acid due to the sulfur content contained in the exhaust gas.
(2) It is important to keep .DELTA.t.sub.1 a difference in terminal temperature between the saturation temperature t.sub.s of the steam-water mixture and the temperature T.sub.1 of the exhaust gas in the evaporating section 12 to 15.degree..about.20.degree. C. for making proper the size of the exhaust gas economizer 1.
However, when the temperature t.sub.i of the circulating feed water at the inlet of the preheating section 11 of the exhaust gas economizer 1 is held at a predetermined temperature (normally 130.degree.-140.degree. C.), if the difference .DELTA.t.sub.1 in terminal temperature at the evaporating section 12 of the exhaust gas economizer 1 is set at a prdetermined temperature (15.degree.-20.degree. C.), it becomes impossible to satisfactorily lower the exhaust gas outlet temperature at the outlet of the exhaust gas economizer 1, so that the rate of heat recovery of the exhaust gases cannot be improved, and, when the temperature T.sub.0 at the exhaust gas outlet is lowered to a tolerance value to improve the rate of recovery of the exhaust gases, such a disadvantage has been presented that it is unavoidable to lower the feed water temperature t.sub.i at the inlet of the preheating section 11 of the exhaust gas economizer to a level where there exists a possibility of the sulfuric acid corrosion.